I think it was Larry Constantine that really hated them. As he put it, when you are writing, you should always be thinking about your next words, but the squiggles draw your attention to the words you have already written. They shout at you, "Hey, listen! Do you really think you can spell? What's this 'fatouos' thing you've just written?" and will keep bothering you until you stop and go back to click on the undersquiggled word to fix it. They are basically a primitive form of Clippy.
Word having two modes, like vi, would solve this. In the writing mode, it never bothers you with anything, just lets you write. As soon as you press the button to switch to the editing mode, it is free to bombard you with squiggles and AI suggestions.
Just as a casual test... I opened up Microsoft Word (online version). There's a button on the Review ribbon labelled "Spelling & Grammar", click that and the realtime, inline suggestions and squiggles are off no matter how many errors are present... click that button again and they're there.
So there are two modes... and have been for as long as I can remember (maybe since automatic spell check was there) and it is just a button press.
Now knowing that it's there... well... how many people review feature documentation these days, especially for something that is "feature rich", like Word?
Do you feel the same way about real-time syntax highlighting?
No, but writing VBA in Office was even worse than the squiggles, because it would show a modal error message if the line you'd just written contained a syntax error.